Guarded: Security staff search a driver in Olympic uniform and his car in Sochi. Russia has defended its efforts against fears its security measures may be inadequate. Photo: AP

As many as 10,000 Americans were expected to visit the Black Sea resort of Sochi for the games, which will begin on February 7, said Obama administration officials, who cited what they called an increase in terrorist threats before the Olympics.

Mr Hagel said he offered "assistance in any way we can help" with security at the games during a recent conversation with his Russian counterpart, Sergei Shoigu.

Security concerns: Chuck Hagel says the US has offered help to Russia for the Olympics. Photo: AP

"As of right now, Russians have not requested any specific assistance," Mr Hagel said.

The US has said it would station two navy ships in the Black Sea during the Games to respond to any emergency.

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Russian authorities have sought to dispel concerns that security may be inadequate after Islamic extremists claimed responsibility for suicide bombings that killed 30 people last month in Volgograd, about 700 kilometres from Sochi.

President Vladimir Putin discussed Olympics security with his US counterpart, Barack Obama, during a phone call on Tuesday.

The US State Department issued an updated travel alert on Friday advising US citizens of the potential threat of terrorist attacks on the Games.

The alert, which replaced one issued January 10, cited uncorroborated media reports about the possible presence of suicide bombers in Sochi and warned spectators to expect an enhanced security presence.

"There is no indication of a specific threat to US institutions or citizens but US citizens should be aware of their personal surroundings and follow good security practices," the State Department said in its alert.

The US will have enough diplomatic security in Sochi to accompany all 230 American athletes to all sporting venues at all times, said one of four Obama administration officials who briefed reporters on Friday on a conference call. They spoke on condition of anonymity to discuss the security preparations.

In a sign of heightened sensitivity, US athletes have been advised not to wear their uniforms or show their team colours outside of the official Olympic venues, an official said.

Russia, as the host nation, remained primarily responsible for security, and the increase in threat reports was not unusual for a major event such as the Olympics, another official said.

The US had staffed an operations centre in Sochi with a clear chain of command to respond to any emergency, this official said.

While the officials declined to say how many US security personnel would be on hand, they said it will be in keeping with force levels provided at past Olympics in other countries.

Sochi, which lies west of the Caucasus Mountains, borders one of the most economically distressed regions of Russia, stretching from Chechnya to Dagestan.

A separatist movement in Chechnya grew into an Islamist insurgency that took its fight into neighbouring provinces. The two suspects in the April 15 Boston Marathon bombing, in which three people were killed and more than 260 were injured, were ethnic Chechen brothers who once lived in Dagestan.

While Mr Hagel said the US would be able to extract US citizens in an emergency, a third administration official said no specific evacuation plan had been developed for the Olympics.

Even though the Pentagon was doing planning and researching the military assets that could be used, evacuations of US citizens were typically co-ordinated by the State Department and would not necessarily involve military aircraft, that official said.

Army General Martin Dempsey, chairman of the Joint Chiefs of Staff, has discussed with his Russian counterpart technology that could be made available to detect homemade bombs, the third official said. No formal request for such equipment was made and none was offered, the official said.